Accelerated weathering devices are used by a number of industries to test a product's resistance to outdoor environments. Often, a manufacturer of a given product will ensure that the product will endure outdoor weather for a specified lifetime. The product must be able to withstand temperature cycles, moisture stresses such as humidity, condensation, and rain, as well as exposure to terrestrial solar radiation. These are important stresses for producing degradation of products used outdoors, and simulation of these stresses is essential for devices that are used for laboratory accelerated weathering tests. Although all three stresses act to produce degradation, exposure to solar radiation is one of the more influential factors in weathering because ultraviolet rays generally tend to break down polymers and other materials over time.
Accelerated weathering devices using an artificial light source have advantages because the use of the devices is not subject to climatic and seasonal variations of temperature, moisture, and terrestrial solar radiation. Among the more difficult tasks in the manufacture of accelerated weathering devices is to provide a spectral power distribution in the artificial light that matches closely to that of natural sunlight on earth. By matching or closely approximating the spectral power distribution of natural sunlight, the results of exposure to the accelerated weathering device will more closely approximate effects of real world exposure.
Approximating spectral power of sunlight is typically accomplished by passing illumination from the artificial light source through one or more optical filters. If this approximating is not done effectively, failure modes observed experimentally in products under test may not match real world failures. Wavelengths of light that are present in artificial light when passed through optical filters and not present in terrestrial sunlight have been found to change the balance of degradation and stabilization reactions. To the extent possible, these wavelengths of light should be eliminated from test protocols. Typical optical filters for accelerated weathering devices, as demonstrated with respect to the comparative examples, pass illumination that does not advantageously approximate sunlight. For example, many filters pass more amounts of ultraviolet radiation at wavelengths less than 290 nm than in actual terrestrial solar radiation, which has only very small amounts of radiation at wavelengths less than 290 nm. Radiation at these wavelengths can cause some materials to fail prematurely, such as polyethylene terpthalate and polyurethanes with phthalate containing polyols. In addition, many filters that do not pass this short wavelength radiation also insufficiently transmit light within the solar spectrum, particularly light at or about 310 nm and can lead to slower degradation rates or longer test times.
To compound the difficulty of manufacturing a suitable optical filter, the optical filters used in accelerated weathering devices are subject to harsh light intensity, thermal and moisture loads. Optical filters must be durable and stable for long periods of time in harsh environments. A filter for an accelerated weathering device may not be selected simply by spectral characteristics alone. Rather, an effective optical filter for an accelerated weathering device must have spectral characteristics that match sunlight and be durable enough to withstand use in the accelerated weathering device.
A significant need exists to provide optical filters for accelerated weathering devices that allow these devices to more closely match the spectral power distribution of terrestrial solar radiation while accommodating the harsh conditions within such devices for an acceptable lifetime.